In prior art cellular radio telephone systems, a geographical area is covered and served by breaking the area into a plurality of small zones or cells. A large land area can be covered by a plurality of connected cell areas. A mobile telephone switching office (MTSO) is provided for each area and is connected to a plurality of base stations, each covering a cell within the area.
FIG. 1 is a diagram illustrating a conventional cellular radio telephone system. Referring to FIG. 1, the user of cellular radio telephone 101 usually travels within an area 102 and is assigned an identification (ID) number for the user's cellular radio telephone which is registered in a corresponding MTSO 103 which covers the area. The area is called a home area and the user is called a home area user. If the user travels out of area 102 and enters another area, the new area is called a roam area and the user is called a roamer in the new area. User fees for calls from the cellular radio telephone via the MTSO covering the roam area are higher than for calls via the MTSO covering the home area. It should be noted that, when a call is made from a cellular radio telephone, the ID number for the cellular radio telephone is transmitted to the MTSO covering the area where the cellular radio telephone is located and the MTSO can therefore distinguish roamers and home area users by checking whether the transmitted ID number is registered in the MTSO.
Similarly, the radio telephone may check within which of the home area or roam area the telephone is located on the basis of system identification (SID) information transmitted from the MTSO, and provides a display as to whether the user is presently a home area user or roamer. The display is very useful to the user because the user may know in advance whether the user fee would be higher owing to the roamer usage.
It is thus proposed to provide the radio telephone with a service area confirmation display function.
An example of the operation of the conventional radio telephone having the service area confirmation display function will now be described. When the apparatus is turned on, the operational state of the circuits in the apparatus are initialized. Then, a control channel (a paging channel, hereinafter called "P-channel") for receiving control signals, such as an incoming call signal, is selected. The P-channel is selected out of the plurality of P-channels, which are dedicated by the MTSO, by measuring the received signal strength thereof and finding the P-channel having the strongest received signal strength. Once the selection of P-channel is completed, the P-channel is set in the receiver and word synchronization is acquired. Thereafter, the reception standby state is maintained. In the reception standby state, the P-channel is reestablished at intervals of five minutes.
System information is detected out of the control signals transmitted through the P-channel, and system identification data through the P-channel (SIDp) included in the system information is stored in the apparatus. The SIDp is compared with a home system identification data (SIDH). The SIDH is a system identification of the system to which the user's apparatus belongs. The SIDH is prestored in an identification data memory (ID-ROM). When both identification data SIDp and SIDH coincide, it is determined that the apparatus is present within the home area. On the other hand, if both identification data SIDp an SIDH do not coincide, it is determined that the apparatus is present in a service area another than the home area, i.e., a roam area system, and an LCD display device displays, e.g., "ROAM", indicating that the apparatus is outside the home area. Accordingly, the user may confirm whether the user's apparatus is located within the home area.
The conventional cellular radio telephone apparatus, however, has the following problem to be solved. In the conventional apparatus, whether the user's apparatus is present within the home area is determined on the basis of the system identification data transmitted from the base station over the P-channel established in the reception standby state. Thereafter, the area display corresponding to the determination result is retained until the SIDp is updated by reestablishment of the P-channel. Consequently, even if the user's apparatus, which may be installed in a car, moves from the home area to a roam area in a short time, the area display made prior to the movement of the user is unchanged. That is, the area display indicates that the user's apparatus is present within the home area. Although the user's apparatus has been actually moved to the roam area, the user's apparatus wrongly indicates to the user that the user is a home area user and a call may be placed in this situation. This results in an undesirable situation wherein high charge is incurred unknowingly to the user.